1. Field of the Invention
The present invention relates to a signal processing circuit, and particularly to a circuit that drives a liquid crystal device in a liquid crystal display device.
2. Description of the Related Art
As shown in FIG. 4, a liquid crystal driving circuit is formed comprising a digital circuit and an analog circuit.
The digital circuit includes a shift register 101 to which data is input column-by-column, data registers 102 that receive data to be displayed from the shift register 101 and that temporarily store the received data, and data latches 103 that hold the data which is being displayed.
The analog circuit includes D/A (digital-to-analog) converters 104 that convert digital data to analog data based on a predetermined relationship, and buffers 105 for power amplification of the converted analog data to drive liquid crystal devices.
A liquid crystal display device has the structure shown in FIG. 5, including a source driver 200 that outputs and supplies display data to a liquid crystal device 203 connected with each column line, and a gate driver 201 that outputs a control signal to a gate of a transistor connected with a desired row line and that supplies the display data to the liquid crystal devices corresponding to the row line to be displayed and thereby writing the display data to the predetermined liquid crystal devices. Due to their low-power-consumption, liquid crystal display devices have recently seen increased usage in portable devices such as cellular phones. In order to support smaller portable devices, the demands for liquid crystal display devices having even lower power consumption have increased.
In the conventional liquid crystal driving circuit such as shown in FIG. 4, the power consumption of the analog circuit is higher than the digital circuit. In particular, the power consumption required for digital-to-analog conversion of the D/A converters 104 and power amplification of the buffers 105 to drive the liquid crystal devices occupies 70% to 80% of the overall power consumption of the liquid crystal display device.
It is therefore necessary to reduce the power consumption of the D/A converters 104 and the buffers 105 in order to efficiently achieve low power consumption of the overall liquid crystal driving circuit.
In one known mechanism, the operations of the buffers 105 and the D/A converters 104, which require high power consumption; are turned on or off by a source driver according to an external signal (see Japanese Unexamined Patent Application Publication No. 2001-188499). The structure of such a source driver will be described with reference to FIG. 6.
A control signal to provide low power consumption is input to a buffer 106, an inverter circuit 108, and a switch 107 in a standby mode.
In the standby mode, the power supply to the buffer 106 is turned off and the power supply to the inverter circuit 108 is turned on, so that a signal from the inverter circuit 108 is output from the switch 107.
In a non-standby mode, the power supply to the buffer 106 is turned on and the power supply to the inverter circuit 108 is turned off, so that a signal from the buffer 106 (analog grayscale data generated by the D/A converter 109, corresponding to a certain number of gray levels) is output from the switch 107.
Therefore, the operation of the buffer 106 is controlled, thus realizing low power consumption.
However, this structure allows for only chip-based batch control of the output modes of the source driver because of standby-mode control.
In this structure, the most significant bit (MSB) of the input digital grayscale data is output to the inverter circuit 108 in the standby mode so as to output binary data.
Thus, only eight colors are represented by using one-bit (the most significant bit) grayscale data for each of R (red), G (green), and B (blue); whereas, 260,000 colors are represented by using 6-bit grayscale data for each of R, G, and B. The above-described structure provides fewer displayable colors, resulting in degradation in image quality.
It is therefore difficult to provide low power consumption for a normal display without reducing the number of displayable colors and the image quality. Accordingly, what is needed is an improved liquid crystal driving circuit providing low power consumption while maintaining the number of gray levels for the image.